In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with a lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide, in the exhaust port, a restricting member which has at its end, disposed adjacent the exhaust port, a restricting edge which, in the restricted position of the restricting member, is substantially flush with the peripheral surface of the cylinder bore. In this restricted position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The restricting member is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
U.S. Pat. No. 4,399,788, entitled "INTERNAL COMBUSTION ENGINE COMPRISING MEANS FOR CONTROLLING THE AXIAL EXTENT OF AN EXHAUST PORT IN A CYLINDER", discloses a system comprising a valve having a restricting member which is actuated by the exhaust gas pressure developed by the engine in the exhaust gas system. It is said that the exhaust gas pressure in the exhaust system is a variable which depends on all significant parameters for the positioning of the restricting member and that the exhaust gas pressure can be used to control the adjustment of the restricting member.
More particularly, the valve comprises a diaphragm for adjusting the restricting member which is gripped in the valve housing and engages a linkage, which is connected to the restricting member. A pressure-applying duct is connected to the exhaust gas system and is adapted to direct the exhaust gas pressure to the diaphragm. A return spring opposes the action of the exhaust gas pressure on the diaphragm and tends to move the restricting member into its restrictive position. A mechanical switch is also provided which renders the valve responsive to exhaust gas pressure only when the throttle is fully opened for high torque when the engine is operating under full load condition.
One disadvantage of this system is that the exhaust gas temperature is very high. A high temperature resistant diaphragm is thus required. Exhaust gas is also very dirty because of its content of oil and fuel, the oil-fuel mixture condensation and the carbonisation which occurs during ignition. In time, this will inevitably create a malfunction of the valve.
Using a pressure source created in the exhaust system cannot be very accurate because of production tolerances. Exhaust pressure is a function of the tuning characteristics of the exhaust system and will be affected by the variations during production. Exhaust gas temperature will also affect the pressure and cause variations. Another disadvantage of this prior art system is that the spring necessary to maintain the restricting member in the restricted position must be calibrated precisely in order to obtain optimum opening point of the restricting member. Because a transitional zone between the exhaust pressure which initiates the movement of the restricting member and the exhaust pressure for which the exhaust port is fully exposed is created by the spring rate, a reduction in engine efficiency occurs during this transitional period.
Because this prior art system uses only exhaust gas pressure to actuate the restricting member and raise the opening of the exhaust port, it caters only to the engine performance when it is operating under full load condition. The exhaust port height adjustment is desirable for all engine load conditions at high revolutions (RPM). The benefit of exhaust port height adjustment is that above a specific combination of RPM and load condition (throttle opening), the engine loses power unless the exhaust gas are able to escape the combustion chamber earlier. The prior art systems is simply unable to prevent power loss for 50% throttle opening combined with high revolutions which is a situation that occurs as often as wide open throttle combined with high revolutions. The exhaust gas pressure varying widely as a function of engine load, RPM, operating mode, ignition timing and exhaust pipe characteristics, and the prior art system being dependant on a single spring calibration for activating the restricting member, it is simply impossible for the prior art system to cater to a variety of RPM-load condition combinations.
Prior art systems using exhaust gas pressure encounter a further problem of hysteresis. When the restricting member opens the exhaust port, the exhaust gas pressure rises slightly which in effect, changes the closing point of the restricting member when the engine RPM falls back down. This phenomena alters engine performance negatively given that the activation point of the restricting member should be at the same RPM in acceleration or in deceleration.
Finally, this system requires the use of a larger diaphragm for two-stroke engines producing a lower pressure in their exhaust systems in order to generate the force needed to actuate the restricting member from the restrictive height position to the full flow height position and reach the goal of increasing engine efficiency.
It is also known to use a valve having a restricting member which is actuated and controlled by the pressuring mixture developed by the engine in the upper part of the cylinder. These systems have a major disadvantage in that the pressurized mixture from the cylinder is very hot and very dirty due to the carbonisation occurring during ignition. This can cause the restricting member and/or the measurement device to become clogged with soot. These systems also generate a transitional zone between two pressures where the efficiency of the engine is not optimum. These systems also use a calibrated spring for actuating the restricting member thereby encountering all the problems stated for the exhaust gas pressure activated systems.
Consequently, these prior art systems, in using a specific pressure as the sole controlling element for actuating the restricting member are less than optimum because they are based on a variable that is dependent on too many other variables. These prior art systems are limited to a very specific load condition. Even in that specific load condition, the movement of the restricting member is not initiated according to different engine speeds (RPM) as it should be, but according to the pressure created in the exhaust pipe or in the upper part of the cylinder which are variables that only partially depend on engine speed amongst other variables.
Moreover, these systems, because they operates according to a specific actuating pressure, require the use of different calibrated return springs for engines developing different power outputs.
Thus there is a need in the industry to provide an exhaust port height adjustment system that alleviates the problems encountered by prior art system and is adapted to cater to any and all engine load conditions.